The present invention relates to a method for selectively strengthening portions of a steam turbine rotor without increasing susceptibility to stress corrosion cracking (xe2x80x9cSCCxe2x80x9d) along the rotor. More particularly, the invention relates to a heat treatment process enabling higher than normal strength conditions at one or more selected axial locations along the rotor without a net increase in susceptibility to stress corrosion cracking.
In order to increase the overall thermodynamic efficiency of steam turbines, the length of the airfoils extending radially from the rotor have been increased, particularly in the last stage. As the airfoil length increases, so does the local stress on the rotor. The airfoil lengths, of course, vary with axial position along the rotor. Consequently, the last stage airfoils experience the highest loading and therefore require increased rotor strength at that axial location relative to the strength of the rotor at other axial locations.
As the strength of the rotor increases, however, so too does the susceptibility of the rotor to stress corrosion cracking (SCC). SCC is an environmental phenomenon that occurs when steels and other alloys are exposed to moisture, contaminants (such as caustic ions) and applied stress. It can occur in conjunction with pitting or dissolution of the protective oxide cover. SCC is evidenced by small cracks in the metal that branch and propagate. Steam turbines are most susceptible to SCC at the point where saturation occurs and at airfoil attachment locations.
The strength of rotors has been variously increased by applying heat treatment processes uniformly along the entire rotor in order to achieve desired strength characteristics. Rotors have also been fabricated from multiple pieces with certain pieces being stronger than others. That process is inefficient as each piece must be heat treated separately. Various altered heat treatment processes have been applied to rotors but to applicants"" knowledge, not for the purpose of SCC prevention. Differential heating of the rotor during austenitizing processes has been used to produce low fracture appearance transition temperature in the low pressure area and high rupture strength in the intermediate and/or high pressure areas. However, there remains a need for a rotor in which selected areas can be strengthened, e.g., to accommodate longer and heavier airfoils for increased thermodynamic efficiency without substantially increasing susceptibility to SCC.
In accordance with a preferred embodiment of the present invention, a turbine rotor is heat-treated to provide increased strength only at one or more selected axial locations along the length of the rotor. Increasing rotor strength, however, also increases susceptibility to SCC at the locations of increased strength. The locations along the rotor at which the strength is increased are also those which traditionally experience lower SCC due to the local operating conditions. These locations occur not only at axial locations where the longer airfoils are secured, but are generally located at axial positions where the temperature and pressure conditions are at a minimum and locations that are continuously wet during operation. Thus, the increased strength at those selected locations does not increase the net susceptibility of the rotor to SCC. In other words, the susceptibility to SCC in the one or more locations of increased strength may approach the same susceptibility to SCC at rotor locations that are lower in strength and experience adverse operating conditions. This results in substantial uniform susceptibility to SCC along the length of the rotor. The SCC susceptibility is lower than it would be if the strength were increased at all positions along the rotor, including those that experience adverse operating conditions. This new rotor fabricating process enables use of longer and heavier airfoils at locations of increased strength without increased susceptibility to SCC and therefore provides rotors which reach higher thermodynamic efficiencies in low pressure steam turbines.
To accomplish this, a preferred embodiment of the present invention provides a method in which the monolithic steam turbine rotor is first austenitized at a uniform temperature, e.g., 840xc2x0 C., over a period of time and subsequently quenched. The rotor is then differentially tempered. That is, the furnace used for the tempering is divided into regions which can be heated to different temperatures. A lower tempering temperature is applied in those regions which heat the rotor at the axial location(s) requiring increased strength. Thus, only those regions of the rotor requiring increased strength are heated to a lower temperature. Since those regions also coincide with the axial locations along the rotor which do not have high susceptibility to SCC, there is no net increase in susceptibility of the rotor to SCC notwithstanding the increases in strength at the one or more axial locations.
In a preferred embodiment according to the present invention, there is provided a method of fabricating a rotor for turbomachinery, comprising the steps of identifying at least one axial location along the length of the rotor requiring a higher strength condition than an axially adjacent location along the rotor and a reduced susceptibility to stress corrosion cracking in service and differentially heating the one axial location and an adjacent location along the rotor, respectively, during tempering to impart higher strength to one axial location in comparison with the strength of the adjacent location whereby a higher strength condition is achieved in one axial location without substantially increasing the susceptibility of the rotor to stress corrosion cracking.
In a further preferred embodiment according to the present invention, there is provided a method of fabricating a rotor for turbomachinery comprising the steps of identifying at least one axial location along the length of the rotor requiring higher strength than the axially adjacent location along the rotor, during an austenitizing process applied to the rotor, substantially uniformly heating the rotor along its length to obtain a rotor of substantially uniform strength throughout its length and, subsequent to austenitizing the rotor, differentially tempering the rotor to relatively increase the strength of the rotor at one axial location in comparison with the strength of the rotor at the axially adjacent location and without substantially increasing the net susceptibility of the rotor to stress corrosion cracking.
In a still further preferred embodiment according to the present invention, there is provided a process for producing a rotor for a turbine comprising the steps of (a) austenitizing the rotor in a furnace over a predetermined time period, (b) quenching the austenitized rotor and (c) tempering the rotor at different axial locations therealong to different temperatures over a predetermined time period without increasing the susceptibility of the rotor axial location tempered at a lower temperature to increased stress corrosion cracking beyond the susceptibility to stress corrosion cracking of adjacent axial locations tempered at a higher temperature.
In a still further preferred embodiment according to the present invention, there is provided a rotor for use in turbomachinery comprising a rotor body having a higher strength at a selected axial location therealong in comparison with the strength of the rotor body at an adjacent axial location, the susceptibility of the rotor body to stress corrosion cracking at the selected axial location being substantially no greater than the susceptibility of the rotor body to stress corrosion cracking at the adjacent axial locations.